The present invention relates to an optical disk drive for driving a compact disk or similar optical disk and, more particularly, to an optical head capable of recording and reproducing data out of optical disks each having a substrate of particular thickness.
It is a common practice with an optical disk to form a transparent substrate having a preselected thickness in order to protect the recording surface of the disk of the order of microns. Rays issuing from an optical head form a fine spot on the recording surface of the disk by being transmitted through the transparent substrate. Because the resulting converged beam propagates through the transparent substrate which is parallel and flat, wavefront aberration occurs in the beam in accordance with the substrate thickness. The wavefront aberration refers to the deviation of the equiphase wave surface of the converged beam from a spherical surface. Should the wavefront aberration increase, the diameter of the converged spot would exceed a diffraction limit and would thereby degrade the recording and reproducing characteristic. In light of this, a lens included in the optics of an optical head is so designed as to correct the wavefront aberration, so that a fine spot close to the diffraction limit can be formed on the recording surface.
Generally, the substrate of an optical disk has a particular thickness matching with a desired application. Because the wavefront aberration stated above depends on the thickness of the substrate, the conventional optical head cannot record or reproduced data in or out of optical disks other than one having a particular substrate thickness. That is, the conventional optical head cannot meet the demand for the recording and reproduction with two or more different optical disks having different substrate thicknesses.
To solve the above problem, a mask area may be formed on an objective lens, as proposed in the past. This, however, brings about a problem that the mask area existing on the lens at all times reduces the optical efficiency. In addition, the mask area reduces the numerical aperture applicable to the head.
Technologies relating to the present invention are disclosed in, e.g., "A Compact-Disk-Compatible Digital Versatile Disk Pickup Using Annular Mask" by Chul Woo Lee et al, Jpn. J. Appl. Phys., Vol. 36 (1997), pp. 486-490 and Japanese Patent Application Laid-Open Nos. 3-52132, 5-6569, 5-73945, 6-203405, 7-57271, 7-65409, 7-98431, 7-302437, and 8-77594.